In the usual installation of an oil burner, a fuel filter assembly with a replaceable filter element is employed. This filter assembly is installed in the fuel supply line of the oil burner and all the fuel oil supplied from the storage tank to the oil burner must pass through the filter assembly. The flow of fuel oil is intermittent, since the pump is turned on and off by a thermostat and possibly other controls. It is generally accepted good service practice to replace the filter element yearly. A typical replacement filter element includes a wire mesh tube or spindle on which the filter element, usually a vertical stack of die cut felt washers, is installed. The tube protudes from the upper and lower ends of the stack of felt washers and cooperates with structures on the top and bottom of the filter assembly to align the element within the filter housing. That type of filter element creates many loose particles of filter material which may be shed from the filter and carried along by the filtered fuel. These loose particles may clog the oil burner spray nozzle, which the filter element is designed to protect. This presents the field serviceman with an annoying dilemma, because he may not know that the clogging of the nozzle is caused by material from the filter element.
To maximize the surface open to contact by the incoming fuel, it is common to build the stack with felt washers of alternately large and small outside diameters. While this does increase the filtering surface, it also creates horizontal surfaces that collect dirt and impurities, thereby shortening the effective life of the filter element.